In this blog I will show you how to interface an Atari-style joystick to the Altera DE1 FPGA board running a Spectrum implementation, how to change the ROM to enable you to input some game-cheat pokes and a few games I eventually completed using this setup....continue reading "ZX Spectrum ROM mods"

In the last article I presented a different way of architecturally modelling a Zilog Z80 processor. It is time to do something really useful with it and what could be better than reliving the past for a moment? Let's recreate an old computer and load in and play some games!...continue reading "ZX Spectrum on FPGA using A-Z80 CPU"

This design would not be possible without Ken Shirriff who reverse-engineered some major portions of Z80 from the picture of a die. These are the portions of the A-Z80 design that are based on his work:

In the first post I described the sequencer, a circuit that provided discrete timing signals to space operations apart. In the second post I mentioned the Timing matrix that was run by these signals and orchestrated a dance of control signals in time.

The sequencer is "the heart" of a CPU. It gets the external clock which in turn toggles two rows of flip-flops that generate machine cycles (M-cycles) and clock periods (T-states)....continue reading "A Z80 : The Heart"

A-Z80 is a conceptual implementation of the venerable Zilog Z80 processor targeted to synthesize and run on a modern FPGA device. It differs from the existing (mostly Verilog) Z80 implementations in that it is designed from the ground-up through the schematics and low-level gates....continue reading "A Z80 From the Ground Up"